This invention relates to the novel combination of a CRT (cathode-ray tube) and an implosion-protection system and to a novel method for producing this system on the CRT.
A cathode-ray tube comprising an evacuated glass bulb is a mass-produced article of commerce. The bulb usually includes a glass faceplate panel hermetically sealed to the wide end of a glass funnel. A luminescent screen is carried on the inner surface of the panel, and one or more electron guns are housed in a neck attached to the small end of the funnel. Some of the adverse effects of implosion of the bulb can be reduced or eliminated by providing an implosion-protection system around the panel.
Some of these systems include a tensioned metal band or strap completely around the bulb and overlapping itself. In one form, overlapping portions of the band pass through a sheathlike metal clip, and the overlapping band portions and the clip are notched together so as to maintain the tension in the band. The notched-together clip and overlapping band portions are referred to herein as a notched seal. Such systems are described, for example, in U.S. Pat. Nos. 3,220,593 to D. E. Powell et al, 3,382,999 to D. E. Powell et al and 3,845,530 to R. B. Platt.
To tension the band around the bulb in these systems, the metal band and clip are positioned around the bulb, and then the band is drawn up by a tensioning device which essentially holds the band in a clamp near the inner one end and pulls the band near the outer other end. The clip, which plays no part in the actual tensioning step, is notched together with the overlapping band portions therein. This method of tensioning, referred to herein as puller-type tensioning, had the disadvantage that part of the tensioning mechanism must be located between the band and the tube during tensioning. As a result, higher than the needed residual tension must be pulled so that, when the mechanism is removed after notching, the required residual tension of at least 400 kilograms, usually about 450 to 675 kilograms (1000 to 1500 pounds), remains in the band. There is also the further disadvantage that the magnitude of the residual tension is variable, being influenced by factors related to the relaxation of the band when the tensioning mechanism is removed.
Puller-type tensioning was borrowed from the packaging industry where it is widely used for strapping packages. The packaging industry also employs another method, referred to herein as pusher-type tensioning, which involves the clip in the tensioning step. In pusher-type tensioning, one end of the band is made into a "J" shape. The band and clip are positioned around the package with the open side of the "J" towards the package. Instead of clamping the inner one end of the band, the "J" of the band hooks over the clip. Then, with a unitary tensioning mechanism, entirely on the outer side of the band, the clip and the inner one end of the band are pushed while the outer other end of the band is pulled until the desired tension is realized. Then, with the tension applied, the clip and the overlapping band ends are notched together, as in puller-type tensioning.
Pusher-type tensioning has the advantage that no part of the tensioning mechanism is between the band and the package or CRT during tensioning. However, when pusher-type tensioning was applied to a CRT, and the required tension is applied to a band on a CRT by pusher-type tensioning, the "J" straightens out, and the band slips through the clip before it can be notched.